1. Field of the Invention
The present invention relates to the general field of microscopy and particularly the field of fast Fourier-infrared (FT-IR) microspectrophotometry.
2. Description of Related Art
Copending U.S. patent application Ser. No. 707,231, now U.S. Pat. No. 4,653,880, title Reflective Beam Splitting Objective, discloses an aperture beam splitter formed entirely with mirror optics. The mirror optics can focus radiant energy having greatly differing wavelengths such as from the visible light region to the infrared. The beam splitter forms an integral unit with the imaging objective of the microscope. In the preferred embodiment, an intercepting mirror is positioned close to the secondary mirror of a Cassegrain reflective objective so as to divide in half the reflective surface of the secondary mirror. The intercepting mirror reflects half of a beam of incident radiant energy to the secondary so that energy from half of the aperture of the primary mirror forms an image at an image sample plane. The other half of the primary mirror collects the radiant energy from the surface of the sample. The arrangement of mirrors enables the reflective beam splitting objective to obtain a 50% throughput efficiency. Little if any of the image information about the sample is lost and the quality of the image is adequate for FT-IR microspectrophotometry. The reflective beam splitting objective, unlike conventional aperture beam splitters, eliminates the complexity and expense associated with collimating optics. Rather than collimating the radiant energy, the reflecting beam splitting objective receives radiant energy that is either converging or diverging.
The foregoing aperture beam splitter has several disadvantages. The intercepting mirror must be positioned in a confined space within the focusing objective. The relatively small physical dimensions of a microscope objective severely limit the physical size of the intercepting mirror. Further, the confined physical space within the objective makes aligning the intercepting mirror most critical. The difficulty associated with obtaining and maintaining the alignment increases the cost of the objective beam splitter. The integral structure of the reflective beam splitting objective requires that each microscope objective contain a properly aligned intercepting mirror. Commercially available reflecting objectives therefore cannot be used without expensive modification.